It is known that rotating a molten metal or alloy at a given speed allows separation by centrifugal force of the impurities or inclusions from the slag contained in a molten metal bath, the lighter particles being returned to the center and rising to the bath surface, where they may be collected by known means.
In other cases, the centrifuging process allows elimination of certain specific alloy constituents from the peripheral region of the ingot for the purpose of reducing the surface hardness of products meant for lamination.
To date, rotation has been mainly achieved by mechanical means in that the ingot mold has been made to rotate as well as the sum of the casting supports about the casting axis; this implies heavy and costly equipment.
Again it has been proposed to achieve a local rotation of the molten metal by means of a rotating magnetic field by placing an electromagnetic rabbling system such as described in French Pat. No. 2 211 305 in the path of the continuous casting and some distance below the ingot mold.
This system allows rotation of the liquid contained inside the ingot when the latter is solidifying and also allows an improvement in homogeneity and crystalline structure of the ingot core.
Such equipment is already found in many continuous steel casting systems, and its range of activity is about 3 meters vertically below the ingot-mold level, in a zone generally termed "secondary cooling zone". At the level of this secondary cooling and on account of ample sprinkling of the ingot from sprinkler banks, the thickness of the solidified wall may reach several centimeters. Therefore the electromagnetic rabbling effect (centrifuging) comes too late to be useful at the ingot periphery.
Now certain steel-and-iron metallurgical applications such as for instance the fabrication of weldless tubing from solid ingots make it desirable to achieve ingots with very high surface quality so as to avoid tube cracking during lamination.
In fact, cracking is initiated because of the slag inclusions retained in the ingot wall at the onset of solidification, i.e., when the metal is in the ingot mold, which represents the primary cooling zone upon leaving which the required solidified wall thickness for ingot strength varies from several millimeters to one centimeter depending on ingot size.
If the product leaving the inductor described in the above cited French patent is to be laminated, the outer ingot crust must be rid, by grinding, of the visible impurities; this is a long and costly labor.
On the other hand, if the molten metal is to be rotated magnetically at the level of the ingot-mold, of which the casting conduit generally is made of copper, or of a copper alloy, and which is quite thick, then this casting conduit opposes the penetration of the magnetic field into the molten metal. The inductor described in the above cited French patent does not allow generating a rotating field with properties sufficient to achieve such rotation, because the insulated water-resistant wires located in the recesses do not allow achieving the proper magnetic field intensity on account of the excessive large volume required by the insulation.
Lastly, when a coil of the type described in the above-cited French patent is used, and there is an accident to the ingot-mold, a long time is required to put the inductor coils back in operation.